The present invention relates to a digital control circuit in which a clock pulse is counted to measure time and when a predetermined period of time has been reached, control for driving a load is effected, and particularly to a combustion control circuit for controlling combustion of a burner, such as for a water heater, an air heater, etc.
The combustion control circuit of the kind as mentioned above is described in, for example, U.S. Pat. No. 4,145,179 and International Laid-open No. WO 80/01604 (International Application No. PCT/JP80/00008, U.S. patent application Ser. No. 224,289).
The configuration and functions of the combustion control circuit of this kind will be explained first by referring to FIGS. 1 and 2. An integrated circuit 1 for combustion control is provided with a pair of electric supply input terminals 2a and 2b, a clock pulse input terminal 3, a flame detection signal input terminal 4, a temperature detection signal input terminal 5, a blower driving signal output terminal 6, an ignition circuit driving signal output terminal 7, a fuel valve driving signal output terminal 8 and a safety device driving signal output terminal 9. The integrated circuit 1 is composed of a timer 11 for counting clock pulses to measure time, and a control circuit 12 responsive to input signals applied to the input terminals 4 and 5 and input signals from the timer 11 to produce driving signals from the output terminals 6, 7, 8 and 9 at appropriate timings or with a proper sequence.
Reference numeral 13 denotes an a.c. source, 14 denotes a d.c. source circuit, 15 denotes an operation switch, 16 denotes a diode, 17 and 18 denote voltage dividing resistors, and 19 denotes a noise-killing capacitor.
The operation of the thus arranged circuit will be described by referring to FIG. 2. In FIG. 2, the left hand numerals correspond to those used in FIG. 1. When the operation switch 15 is turned on and the input terminal 5 receives at time t.sub.o an input signal which indicates that the temperature of a load has been lowered below a preset value, the control circuit 12 causes the timer 11 to begin to measure time. Clock pulses obtained by half-wave rectifying the a.c. power from the a.c. source 13 are successively applied to the input terminal 3. The timer 11 begins to count the successive clock pulses received by the input terminal 3. That is, a period of prepurging is started and the control circuit 12 produces, at this time, a driving signal from the output terminal 6 to instruct the initiation of the prepurging operation.
When the count of the timer 11 has reached a predetermined value, that is when a preset period of time has been reached, the timer 11 sends a control signal to the control circuit 12 so that the control circuit 12 produces driving signals from the output terminals 7 and 8 to cause an ignition circuit (not shown) and a fuel valve (not shown) to start the igniting operation at time t.sub.1. The timer 11 continues its counting operation even if the time t.sub.1 has been reached. That is, the timer 11 begins to measure a safety period of time from this time t.sub.1. If a flame detection signal indicating the existence of a flame is received at the input terminal 4 at time t.sub.2 within the safety period, the control circuit 12 stops the producing driving signal from the output terminal 7 so that the combustion is performed in a normal combustion state. If the flame detection signal disappears, that is if the flame is extinguished, in the normal combustion state, the combustion is restarted from the above-mentioned prepurging operation. Further, if the load temperature exceeds a predetermined value so that the temperature detection signal disappears at the input terminal 5 in the normal combustion state, the control circuit 12 stops the combustion.
In case no flame detection signal arrives at the input terminal 4 during the safety period, the operation is repeated from the beginning of the prepurging operation, or the control circuit 12 stops producing the driving signals from the output terminals 6, 7 and 8 at time t.sub.4 and produces, at this time, an alarm signal from the output terminal 9 to stop the combustion operation.
In case any one of the input terminals 8, 4, 9 and 3, for example the input terminal 3, is grounded or a lead wire thereto is broken, intentionally or accidentally, during the safety period, for example at time t.sub.3, the safety period will be prolonged infinitly due to the disappearance of a clock pulse, resulting in serious danger because fresh fuel may be released if ignition has not been attained (assuming that the d.c. source is being supplied).
As a countermeasure of this, a second timer may be provided in addition to the timer 11 of the integrated circuit 1 or alternatively a further clock pulse input terminal may be provided in addition to the terminal 3. Each of these cases is, however, expensive.